Inexpensive photodetectors that measure the intensity of light in a number of wavelength bands are required in a number of devices. For example, light sources that utilize red, blue, and green LEDs to generate light that is perceived as being a particular color often utilize photodetectors in a servo loop that maintains the output of the LEDs at predetermined levels to compensate for aging. The photodetectors are used to measure the output of each LED by measuring the light generated by the LEDs in each of three spectral bands. A controller varies the average current to each LED such that the measured outputs are maintained at target values determined by the perceived color of light that is to be generated.
Each photodetector typically consists of a photodiode that is covered with a pigment filter that limits the photodiodes response to light in a corresponding band of wavelengths. The signal from the photodiode is determined by the incident light, the bandpass filter characteristics of the pigment and various background signals that are present independent of the intensity level of the light reaching the photodiode. The light-independent signals are often referred to as the “dark current”. The errors generated by the dark current can be removed by measuring the output of the photodiode when no light is present and then subtracting the measured signal value from the signals generated by the photodiode in the presence of light. For example, an additional photodiode that is covered by an opaque layer that blocks all light can be included in the photodetector. The signal from this photodiode is then subtracted from that generated by the photodiodes that are covered with the various pigment filters.
Unfortunately, the pigment filters that are available for use in inexpensive photodetectors have significant transmission bands in the infrared portion of the optical spectrum as well as the bands in the desired visual region of the spectrum. In many cases, the light sources of interest also include light in the infrared region of the spectrum; hence, the signal generated by a photodiode that utilizes one of these pigment filters can include an unwanted infrared background signal if the light source includes a significant amount of infrared light. The infrared light is either generated by the light source being controlled or by background ambient light sources that introduce light into the input to the light sensor.
In some prior art systems, an infrared blocking filter is provided over the various pigment filters to block the unwanted infrared signal. However, the additional filter increases the costs of the light sensor. In addition, the infrared filters are less than 100 percent transparent in the visible region of the spectrum, and hence, the added filter over the pigment filters attenuates a portion of the light of interest, and hence, reduces the sensitivity of the light sensor.